Microwave oscillator



March 8,1960

Filed March 28, 1957 W. H. THON MICROWAVE OSCILLATOR 3 Sheets-Sheet l INVENTOR. WILLIAM H THON AT T OR N E Y.

March 8, 1960 w. H. THON 2,928,020

MICROWAVE OSCILLATOR Filed March 28, 1957 3 Sheets-Sheet 2 INVENTOR. WILLIAM H. THON ATTORNEY.

March 8, 1960 w. H. THON 2,928,020

MICROWAVE OSCILLATOR Filed March 28, 1957 3 Sheets-Sheet 3 INVENTOR. WILLIAM H. THON O yaw ATTORNEY.

United States Patent 2,92s,020 MICROWAVE OSCILLATOR assignments, to Sylvania ,Electric Products 111e,: Wilmmgton, Del., a corporation of Delaware Application March 28, 1957, Serial No. 649,239: 14 Claims. (11. 315-35 9 i This invention relates to space charge devices. and more particularly to microwave backward wave oscillators and/or amplifiers which utilize the interaction betweenan electron stream and an electromagnetic wave.

in such devices, an electric circuit such as a helix or an interdigital filter type circuit having a plurality of' fingers propagates radio frequency electromagnetic waves therethrough at velocities slower than the velocity of light and an electron stream is projected through the electric field set up by the electric circuit by an electron gun at 'oneendof the tube and collected at the other end.

Suitable apparatus for producing a magnetic field longitudinally ofthe circuit to' focus the electronstream usual-' 1y surrounds the envelope of the device. The theoryof operation of such backward wave devices can best be explained by consideration of the build up ofoscillation's. Assuming that a small pulse of noise near a certain frequency f develops on the wave propagating circuit near the collector .end of the tube, some of the energy will propagate along the circuit toward the opposite end (electron gun end) of the tube. 7 This wave will have a phase velocity in the opposite direction, that is, from electron gun end to collector end of the tube. It a small electron current at a velocity just greater than this phase velocityis introduced, average kinetic energy of electrons.

willsbe converted into wave energy, increasing the energy of the circuit wave and resulting in more energy appearing at the electron gun end of the circuit than before the' beam was turned on. .The action thus far described is similar to the action of a forward traveling wave amplifier, except that wave energy and electron-beam energy are traveling in opposite directions; howeverythis backward-wave amplifier is inherently regenerative;

even when the wave propagating circuit is matched perfectly at both ends. The circuit fields atthe electron gun end produce velocity modulation of the incoming electron beam, and this velocity modulation becomes current modulation near the collector end'of the tube.

' The current modulation induces energy in the circuit i n the form of a wave whose phase velocity is nearly "the same as the electron velocity or a wave whose'energy. is traveling back'toward the gun or output end of the" tube. This returned energy is not caused by reflection; if the output load is matched, therelis no circuit energy traveling from the output of the circuit to the internal termination of the circuit. The returned energy produces tributing to the weight of the device.

William H. Thou, Palo Alto, Calif., assignorfby rnesne 2,928,020 Talen e M secured to opposite sides thereof to form a flat envelope,

with interdigital fingers. secured to said tubes internally of the envelope and positioned transversely thereof. An

electron gun capable of producing two parallel sheet beams of electronsis mounted at one end of the enve- -lope, the orientation of the beams relative to the fingers affording interaction between the electrons;and both sides of the slow wave structure.

for' the electron beams. a

As noted above in the brief discussion of the theory of operation-of backward wave devices, for operation i as anoscillator the slow Wave structure. is'terminated. at

the collector end in a manner to eliminate the wave traveling in the forward direction (i.e., from electron gun towardj-fcollector) :on'the circuit. Transmission lines of the interdigital type are diflicult to terminate 'so as to be reflectionless, and heretofore required an extension of considerable length beyond the useful length of the tube to enclose suitable materials and/or structures for absorbing the forward wave. This obviously added to the size and weight of the tube envelope, the increase in 7 length also adding the requirement for magnetic-focusing vide an improved termination for the interdigital struc-' microwaves traveling in the forward" direction. In 18capparatus for a longer electron beam travel, which furtheradded to the overall size and weight of the equip ment.

' Another object'of the invention, therefore, is to proture. of a backward wave oscillator; forabsorbin'g "the cordancewith the invention, a transition is" provided from the interdigital line to another type of transmission line which is folded back along the interdigital line so as' not to extend the length of the device beyond its useful length. A terminating resistive element, which may take a'variety of forms, is placed in the second line, andenclosed within one of the tube'slemployed in the fabrication of the envelope. Thus, apart from the saving in forms the outer conductor of a coaxial transmission line length andspace, the terminating resistor, which might otherwise deteriorate at elevated temperatures or by electron bombardment, is protected by virtue of its enclosure in a vacuum and placement outside the region of the electron beam. 1 m I V I The othermetallic tube ofv the envelope-structure for derivingan output from the oscillator at the electron 'gurrend of the interdigital line. A transition similar to that used for the terminating line couples the gun end further bunching, whichin tu'rn, produces more returnedj" energy.

The slow wave circuits of backward wave amplifiers and oscillators have heretofore taken a variety of forms, including interdigital structures of various kinds, straight finger elements formed either by milling. appropriate slots ina solid block of conducting material, normally copper,

or by stacking a series of laminations shaped and arranged to form the interdigital configuration. This type of construction usually requires a considerable q n o s r fer S po t r the fi ge s. fli bulk of the interdigital line to this coaxial line, the transition occupying insignificant space. Thus,- the two metallic tubes, in addition to forming lightweight structural sup- ,port for-the envelope and the interdigital structure also serve as conductors further to contributeto the compactjjness of the apparatus. Another object of the invention,

therefore, is to provide a backward wave oscillator .or'

amplifier of compact construction. v r

The foregoing and other objects and further features and d a ages w ll be appa a d a better nd standing o the co truct on an op a io of the inye sti nwill belied from the fo low ng de a ed description An end plate closes the other end of i the envelope and serves as th'ecolleo'tor structure of generally rectangular cross-section.

Fig. 3 is a cross sectional View taken along line 3-3 of Fig. 2;

Fig. 4 is a cross-sectional view taken along line 44 of Fig. 2;

Fig, 5 is a cross-sectional view taken alongline 5-5 of Fig. 4;

Fig. 6 is a view similar to Fig. 2 illustrating an alternative construction; and

Fig. 7 is an enlarged fragmentary cross-sectional view illustrating the details of the terminating line of Fig. 6.

Referring to the drawings, and more particularly to Fig. 1, the envelope of the oscillator is formed of a pair of cylindrical tubes 1t? and 12 arranged parallel to each other and to which are joined a pair of similar plates 14 and 16 having turned-in edges, so as to provide an open Tubes 10 and 12, and plates 14 and 16 are formed of nonmagnetic material, stainless steel being admirably suited because of its strength and rigidity even with thin sections and its properties of being readily brazed to form .vacuum tight seams. The envelope is closed atthe right end (as viewed in Fig. l) by an oval-shaped plate 18, having rectangular apertures 18a and 18b therein for the entry of sheet electron beams (to be described in detail), to

which is joined, as by brazing, a section of tube stock which-forms an enclosure 20 for the electron gun structure. As best seen in Fig. 2, the enclosure is closed by an oval end plate 22 having a disc 24 of insulating material supported centrally thereof to serve as a feed-through for conductors 26 by which cathode heating current and operating potentials are supplied to the tube. End plate 22 is further provided with an exhaust tribulation 2 8,

through which theentire envelope may be evacuated and which may besealed by methods available to the art. The opposite end of the envelope is closed by an ovalshaped plate 30, which may also be formed of stainless steel, which is brazed to the ends of plates 14 and 16 and to the tubes, and also serves as the collector for the electron beam. 7

Internally of the envelope tubes 10 and 12 provide structural support for the slow wave circuit of the device which consists of a plurality of elongated fingers of rec-v tangular cross-section extending transversely of the envelope and interdigitally arranged throughout the length of the envelope. More specifically, the slow wave circuit consists of a plurality of fingers, which may have a cross section as shown in Fig. 5, half of which, each designated by numeral 32, are secured, as by brazing-to tube 10 and extend across the envelope to leave a small space 34 between the ends thereof and tube 12, and the other half of which, each designated by numeral 36, are secured to 0 34 and 38 of course, being determinants of the frequency at which the device will operate. As will be shown later, two flat electron beams are projected through the slits 18a and 18b of end plate 18 lengthwise of the tube in close proximity to the upper and lower edges, respectively,

of fingers 32 and 36 to provide interaction between the electric fields between the fingers produced by the waves propagated thereon and the electron streams.

As briefly outlined earlier, for operation of the device as a backward wave oscillator, the interdigital structure must be provided with a termination at one end to absorb the forward waves traveling from the electron gun end toward the collector end, with the output derived from the gun end of the structure. In accordance with the present invention the former objective is accomplished by a second line, joined to the interdigital circuit by a suitable transition, and folded back along the interdigital circuit. To this end, and referring to Figs. 2 and 3 which show one form of termination, the final finger at the collector end of the slow wave structure is secured to tube 12 (this finger being designated 36' to distinguish it from others in the series) and instead of terminating short of tube It) as do the others, extends through a cut-out 41) provided in the inwardly extending portion of the periphery of tube 10 for connection to conductor 42 positioned coaxiallywithin the tube It). The final finger secured to tube 15) at the collector end, designated 32, is secured at the edge of cut-out 4d, and with finger36 provides a wave guiding structure for coupling wave energy propagated toward the collector end to the coaxial line consisting of conductor 42 and tube 10. This coaxial transmission line is terminated by a lossy resistive element, capable of absorbing the energy coupled to the line. In the embodiment of Fig. 2, the two conductors of the line are shunted by a mass of resistive material 44, which may be a carbonized porous ceramic material.

For example, the material 44 may be a porous ceramic rod previously carbonized by impregnation thereof with sugar in solution and heating in a suitable atmosphere to convert the sugar into carbon. The right end of the line is terminated by a shorting plug 46 closely fitted within tube 10 and joined to inner conductor 4-2. Carbonized ceramics of this, or other composition, may deteriorate at operating temperatures, particularly by-volatilization, but it will be noted that in the present construction the material is enclosed in the vacuum envelope thereby protecting it from such deterioration and contributing to the stability of its characteristics. it will also be noted that the resistive material is displaced from the region of the electron stream and consequently not subjected to electron bombardment.

Instead of the shunt termination of Fig. 2, the line may alternatively be provided with a series termination, as shown in Figs. 6 and 7. In this form, the tube is identical with that shown in Fig. 2, except that the final finger 36 is connected to conductor 48, coaxially disposed within tube 10, having a series termination comprising a suitable resistor 50 having stable characteristics when enclosed in a vacuum and subjected'to the operating temperatures of the device. As an example, resistor 50 may be formed of a carbonized porous ceramic of the type described above, and supported on a shorting plug 52 fitted within the tube 10. It will be noted that in this case, also, the lossy resistive material 50 is protected by the evacuated envelope, and displaced from the electron beam.

Again referring to Figs. 2 and 4, a transition similar to that used for coupling one end of the slow wave structure to the terminating line is used to couple the other end of the structure to an output connection. The final finger at the gun end of the interdigital circuit is secured to tube 10, this finger being designated 32", and extends through a cutout 52 provided in the inwardlyextending wall of tube 12, for connection to a conductor 54 coaxially disposed within tube 12. The finger 32", together with the. next adjacent finger 36" provides a wave-guidingcircuit for coupling the backward traveling wave (collector toward gun) to the output coaxial line formed by conduc tor 54 and tube 12.

Thus it is seen that tubes 10 and 12, in addition to separating and supporting plates 14 and 16, and serving supports forthe interdigital fingers, each constitutes vthe outer conductor. of a coaxial line for coupling wave energy from the slow wave structure. The result is a of the electromagnets or permanent magnets close to the path of the electronstreams,thereby reducing the total magnetic flux required to achieve proper focussing, which in tuna-results a reduction in size and weight of the fieldproducing apparatus, and the requirements of the power supply therefor. 1 Inother words, the design and construction'of 'theenvelope effects a cumulative saving is intended for airborne operation.

As best seen in Fig. 6, the left end of the "coaxial line formed by tube 12 and conductor 54 is provided'with a vacuum-tight seal 56, whereby the entire envelope, including tubes 10 and 12, may be evacuated. Seal 56 includes an axial conductive stub 58 threaded at one end 1 into conductor 54 and terminated at the other end by a pin 60. A cylindrical sleeve 62, sealed to the end of tube barns is secured to sleeve 62 and servesas' an adapter f for the connection thereto of commercially-available 'coaxial'connectors. This construction aifords" an'-"air-tight closure for tube 12 yet allows the transmission of microwave energy thereto-with a minimum of attenuation and reflection. i

As noted earlier, end plate 30 also functions as the collector for the electron beam, and to provide for the convenient connection of suitable leads thereto for application of accelerating potentials, a terminal 80 may be conductively secured thereto, as by welding or brazing.

Referring to Figs. 1, 2 and 5, the electron gun for the tube is supported on four posts, 90, 92, 94 and 96 secured to and extending from end wall 18o'f the envelope, the several elements making up the gun being spaced and the posts. The gun is arranged to providetwo sheet beams of electrons, and to that end includes a cathode structure 98 of generally H-shaped cross-section extend" .ing across the wider dimensions of the envelope, thevere tical members 98a and 98b of the structure being hollow 106a and 10612 and enter the slots 18a and 18b in the end plate. The cathode assembly is supported on posts 90, 92, 94 and 96 and is insulated therefrom, with the areas 98c and 98d in alignment with slots 18d and 18b. Assodigital'fingers 32, 36 and closely adjacent thereto, the

electron beams accordingly passing closely adjacent ,to' the exposed edges ofthe. fingers to provide maximum interaction-with the electric fields produced in the spaces between-the fingers by electromagnetic waves propagated thereon. After traversing the length of the envelope, the electron beams are collected by end-plate 30.

From the foregoing, it has been seen that applicant has provided a construction for a backward wave oscillator affording the advantages of reduced weight over prior art devices of this. type, by reduction in weight of the tube structure itself and by an arrangement of parts to reduce the size and weight of magnetic equipment, improved in weight, a factorvery important when'the equipment I 1 insulated from each other by ceramic spacers fitted over -.:sealed envelope formed of conductive material, a linear t accelerated by anode 106 and pass through aperturesciated with the cathode'structure is a conducting plate 104 supported on" the postsand provided with elongated cut-outs 104a and 10412 aligned with areas 98c and 980', respectively, having dimensions such that the cathode structure is electrically separated from plate 104. Plate l'04yis formed to "provide sloping areas 104a and 104d extending upward from, cut-outs 104a and 104b, the

electron-emissive, thereby to obviate the production andacceleration of electrons which are not used.

As best seen in Fig. 5, slots 18a andlSb are symmetrically located on either side of the array of inter- It will be noted that inthis construction only.

coupling between electron stream and slow wave structure, and. reduction in the length of the structure. While a preferred 'embodiment of theinvention has beenshown,

it is to be understood that applicant does not wish to'be 'limited' thereto sincernany modifications can now be made by" ones skilled 'inthe art, and applicant, therefore, contemplates by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention. v

What is claimed is:'

1...In abackward wave oscillator, a slow wave structure for electromagnetic waves comprising a linear array of interdigitally arranged finger elements, an output terminal connected to the last finger element at one end of said linear array, and a lossy coaxial transmission line coupled only to the last finger element at the other end of said linear array and disposed alongside said linear array and transversely displaced therefrom for absorbing waves traveling on said 10w" wave structure toward said other end. r

29A backward wave oscillator comprising, an elongated sealed'envelope, a linear array of interdigitally arranged ,fing'er elements disposed within said enevelope along the longitudinal axis thereof, means at one end of said enline'coupled to said array at the opposite end from said electron-directing means and extending within said envelopetoward said one end of said envelope and lying on the side of said'linear array opposite from the region of said electron stream. 3. A backward wave oscillator comprising, an elongated array of interdigitally arranged finger elements secured to-said envelope and disposed along the longitudinal axis thereof, means at one end of said envelope to direct a stream of electrons along one side of said array, a collector for electrons at the other end of said envelope, and a lossy coaxial transmission line coupled to said array near said collector and extending within said envelope toward said one end and arranged substantially parallel to said longitudinal axisand lying on the side of said 'array opposite from-the region of said electron stream.

4; Backward wave apparatus comprising a sealed envelope including a pair of parallel spaced apart tubes and a pair of spaced plates joined thereto to form anenclosure of generally rectangular cross-section, a slow wave structure for'electromagnetic waves including a plurality of finger'eletnents joined to said tubes and interdigitally arranged in a linear array, a'first conductor disposed within 7 one of said tubes and connected to the last finger element at one end'of said linear array and with said one tube constituting a coaxialoutput connection, and a second conductor disposed within the other of said tubes and connected to the last finger element of the other end of said linear array and with the said other tube consti- P tuting a coaxial line termination for said slow wave structure.

. 5. In a backward wave oscillator, an elongated seale envelope having a longitudinal axis, a slow wave structure I for electromagnetic waves including a plurality of finger elements interdigitally arranged in a linear array along said longitudinal axis, an output connection coupledto the last finger element at one end of said linear array,

'and a lossy coaxial transmission line coupled to the last finger element at the other end of said linear array and extending within said envelope toward said one end with 1 an output connection coupled to the last finger element at one end of said linear array, and a lossy transmission line coupled to the last finger element at the other end of said linear array and disposed within said sealed envelope and extending toward said one end in a direction substantially parallel to said longitudinal axis.

7. A backward wave oscillator comprising an elongated sealed envelope having a longitudinal axis, an interdigital transmission line disposed along said longitudinal axis substantially coextensive with said envelope, means to direct an electron stream along and parallelto said transmission line, electromagnetic coupling means coupled to that end of, said transmission line near said electron directing means, and a coaxial transmission line including wave-absorbent material coupled to that end of said transmission line remote from said electron directing means and arranged within said sealed envelope substantially parallel to said longitudinal axis.

8. A backward wave oscillator comprising an elongated sealed envelope including first and second spaced apart tubes and a pair of spaced plates joined to said tubes and .arranged therebetwe'en, a transmission line for electromagnetic waves substantially coextensive with said en- 'velope including a plurality offinger elements joined to said. tubes and interdigitally arranged in a linear array, means to direct at least one electron stream along and parallelto said vtransmission line, a collector for said electron stream and constituting an end plate of said envelope, a first conductor disposed within said first tube and connected toa finger elementof said array near said electron directing means and with said first tube constituting an output coupling for electromagnetic Waves, a

second conductor disposed within said second tube and connected to a finger element of said array near said collector and with said second tube constituting a coaxial transmission line, and wave absorbent material in said coaxial transmission line to absorb electromagnetic waves propagated on said transmission line toward said collector.

9. A backward wave oscillator comprising, an elongated sealed envelope including first and second spaced apart conducting tubes, a pair of spaced conductive plates joined to said tubes and arranged therebetween and a conducting end plate joined to one end of said tubes and plates, a transmission line substantially coextensive with said envelope including a plurality of interdigitally arranged conductive finger elements secured to said first and second tubes and arranged in an array disposed intermediate said plates, means to direct a fiat stream of electrons along both sides of said array of finger elements toward said end plate, electromagnetic coupling means including said first tube coupled to said transmission line near said electron-directing means, a conductor coaxially disposed within said second tube and connected to a finger element near said end plate and arranged to couple from said transmission line electromagnetic waves propagated toward saidend plate, and wave-absorbent material arranged within the coaxial line formed by said conductor and said second tube. i

8 10. Apparatus in accordance with 'claim9 wherein said wave-absorbent material. is arranged in the annular space between said conductor and said second tube.

11. Apparatus in accordance with claim 9 wherein said wave-absorbent material is positioned in series with said conductor.

12. Apparatus in accordance with claim 9 wherein said wave-absorbent material comprises carbonized ceramic material.

13. A backward wave oscillator comprising, an elongated sealed envelope, a slow Wave structure arranged within said envelope and substantially coextensive there- 7 with, an electron gun at one end of said'envelope for tron gun toward said collector comprising a lossy coaxial transmission line making only an end to end con nection with said slow Wave structure adjacent said collector and extending therefrom alongside said slow wave structure inside said sealed envelope and outside the region of said stream of electrons.

14. A backward Wave oscillator comprising, an elongated sealed envelope including first and second spaced apart conductive tubes and a pair of spaced conductive plates joined to said tubes and arrangedtherebeweteen, a slow wave structure for electromagnetic waves arranged within said envelope and substantially coextensive therewith, said slow wave structure including a plurality of conductive finger elements joined to said tubes and inter digitally arranged in a linear array, an electron gun at one end of said envelope for directing a stream of electrons along said slow wave structure, a collector at the other end of said envelope, a first coaxial line including a first of said tubes as its outer conductor conected to said slow wave structureat the end thereof adjacent said electron gun for coupling from said structure waves traveling in the direction from said collector toward said gun, and means for absorbing unwanted waves traveling in the direction from said electron gun to said collector comprising a lossy coaxial transmission line having the other of said tubes as its outer conductor connected at one end to the end of said slow wave structure adjacentsaid collector and disposed alongside said slow wave structure inside said envelope and outside the region of said stream of electrons.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,647 Lindenblad Apr. 21, 1953 2,588,831 Hansell Mar. 11 1952 2,702,370 Lerbs Feb. 15, 1955 2,727,179 Lally et a1 Dec. 13, 1955 2,748,268 Whinnery May 29, 1956 2,774,006 Field et al Dec. 11, 1956 2,788,465 Bryant et al. Apr. 9, 1957 2,802,136 Lindenblad Aug. 6, 1957 2,808,534 Dallons Oct. 1, 1957 2,829,300 Wilson -.'Apr.- 1, 1958 2,837,684 Unger June 3, 1958 ,355 Epsztein Mar, 31, 1959 FOREIGN PATENTS Great Britain Nov. 18, 1953 

